function a(){
    head.next = {
        function() {
            head.next = {
                function() {
                    head.next = {xxxx}
                    return head.val === val ? head.next : head;
                }
}
            return head.val === val ? head.next : head;
        }
}
    return head.val === val ? head.next : head; // 1 
}